This invention relates to a gel-free process for making functionalized polymers, primarily functionalized anionic polymers which are made using multi-lithium initiators. More particularly, this invention relates to a gel-free process for making polydiene diols.
Functionalized anionically polymerized polymers of conjugated dienes and other monomers wherein the functionalization is terminal and/or internal are known. Particularly, U.S. Pat. No. 5,393,843 describes polybutadiene polymers having terminal functional groups. One of the methods described for making such polymers involves anionic polymerization utilizing a dilithium initiator such as the adduct derived from the reaction of m-diisopropenylbenzene with two equivalents of s-BuLi. Monomer is added to the initiator in hydrocarbon solution and anionic living polymer chains grow outwardly from the ends of the dilithium initiator. These polymers are then capped to form functional end groups as described in U.S. Pat. Nos. 4,417,029, 4,518,753, and 4,753,991. Of particular interest herein are terminal hydroxyl, carboxyl, sulfonate, and amine groups.
It has been observed that when the living polymer is reacted with the commonly available xe2x80x9ccappingxe2x80x9d agents, the polymer in the hydrocarbon solution forms a gel. For purposes of this invention, a polymer gel is defined as a blend of a polymer and a hydrocarbon solvent that has a yield stress, that is, it will not flow unless it is acted on by at least some critical stress. A polymer gel as defined herein will require a significant application of force in order to initiate flow through an orifice. Of particular interest are gels that will not flow under the force of their own weight. The presence of gel that will not flow under the force of its own weight is readily detected by visual observation. This effect is observed by inverting a bottle containing the solution to see whether it flows to the bottom of the inverted flask. Gelled solutions will not readily flow to the bottom of the bottle.
The physical characteristics of these gels make them more difficult to handle in equipment which is designed for moving, mixing, or combining freely flowing liquids, i.e. materials without a significant yield stress. Pumps, reactors, heat exchangers, and other equipment that are normally used for making polymer solutions that can be characterized as viscous fluids are not typically suited to handling polymer gels. Thus, one would expect that processing equipment likely to be found at a manufacturing location that is designed to handle liquid polymer solutions, as defined above, would be ill suited to handling gels of this nature.
Without limiting the invention thereto, we offer the following theory as to why this gelation occurs. We believe that gelation results from the strong association of the xe2x80x9ccappedxe2x80x9d polymer chain ends in the hydrocarbon solvents used, i.e., cyclohexanesldiethylether. In the case of an ethylene oxide capping agent, the polymer chain ends would be lithium alkoxides. In essence, these very polar lithium alkoxide sites interact strongly as they are formed and, in the nonpolar solvent, self-assemble into aggregates having multiple alkoxide centers. The association of alkoxide ends from multiple chains in a single aggregate provides a mechanism for network formation. Since the polymer chains each have two alkoxide ends, having the ends anchored in different aggregates leads to elastic properties, creating a gel as defined herein.
A suggested mechanism for the formation of a strongly associating gel in the case of a polybutadiene diol is as follows:
Lixe2x80x94Li+Butadienexe2x86x92Lixe2x80x94CH2PolymerCH2xe2x80x94Li 
Lixe2x80x94CH2PolymerCH2xe2x80x94Li+2(ethyleneoxde)xe2x86x92

The dilithium initiation technology discussed above has advantages over other technologies used to make functionalized anionically polymerized polymers including polydiene diols and polyols. For instance, U.S. Pat. No. 5,416,168 describes a process which utilizes a monolithium initiator which contains a protected functional center (Protected Functional Initiator) to make a polybutadiene mono-ol. The preparation of the initiator is complicated by the fact that the precursor to the initiator must contain the functional center that is desired in the final polymer and further that this center must be derivatized to make it inert to the chemistry used in making the Cxe2x80x94Li bond in the initiator. Once the Protected functional initiator is prepared, it may be used to polymerize a suitable monomer such as butadiene. This process leaves the protected functional initiator on one end of the polymer chain and a living Cxe2x80x94Li center on the other end of the chain. Optionally, the xe2x80x9clivingxe2x80x9d end of the polymer chain may be reacted with a capping agent. If ethylene oxide is used as the capping agent, then a polybutadiene mono-ol is the product. These steps are then followed by a step of deprotecting the first functionalized polymer chain end. This chemistry frees the functionality on the other end of the polymer chain. Finally, the polymer product must be washed to remove the residue of the protecting agent and the residues of the reagent that were used to remove it.
It can be seen that a dilithium initiation process would be highly advantageous over such a protected functional initiator process in terms of elimination of process steps, cost, etc. The invention described herein is a process for producing terminally functional polymers using the di- or multi- organo alkali metal initiator method. This process for making terminally functional polymers avoids gel formation through the addition of xe2x80x9cscreening agentsxe2x80x9d which block or weaken the association of the polar functional moieties.
This invention relates to a gel-free process for making functionalized polymers. When multi-organo alkali metal initiators are used to make these polymers anionically, the process comprises anionically polymerizing at least one monomer with a multi-organo alkali metal initiator in a hydrocabon solvent and then capping the polymer by adding to the polymer a capping agent that reacts with the ends of the polymer chains such that strongly associating chain ends are formed wherein a polymer gel is formed. The important characteristic of the capping agent herein is that it caps the living polymer and adds a functional group to the polymer chain end which will be strongly associating in the hydrocarbon solvent. The result of the association of the chain ends is that the solution will gel. The final step of the process is adding a trialkyl aluminum compound to the polymer gel which results in a freely flowing solution.
In a second embodiment, the present invention relates to a process for making such polymers which comprises anionically polymerizing them as described and then capping the polymer by adding the above-described capping agent. An aluminum trialkyl is added before or during polymerization or before or with the capping agent (i.e., before a gel can formxe2x80x94prior to any reaction of the alkali metal with the gel-forming functionality).
In the first embodiment, a gel is formed and then removed. In the second embodiment, the gel never is formed because of the presence of the trialkyl aluminum compound.
In a third embodiment, an unfunctionalized polymer is functionalized by lithiation and reaction with a capping agent of this invention, whereby a strongly associating gel is formed. A promoter such as triethylamine or tetramethylethylenediamine is necessary. In a fourth embodiment, an already functionalized polymer is reacted with RLin (or an active Na or K compound) in order to convert to a different functionality. In both embodiments, the gel can be broken by addition of trialkyl aluminum to the gel or prevented by addition thereof prior to the reaction of Li (or Na or K) with the gel-forming functionality.
This invention relates to functionalized polymers and processes for avoiding gel formation, especially when such polymers are made by anionic polymerization using di- or multi-alkali metal, generally lithium, initiators. Sodium or potassium initiators can also be used. For instance, polymers which can be made according the present invention are those from any anionically polymerizable monomer, especially including terminal and internal functionalized polydiene polymers, including random and block copolymers with styrene, polyether polymers, polyester polymers, polycarbonate polymers, polystyrene, acrylics, methacrylics, etc. Polystyrene polymers hereunder can be made in the same manner as the polydiene polymers and can be random or block copolymers with dienes.
In general, when solution anionic techniques are used, copolymers of conjugated diolefins, optionally with vinyl aromatic hydrocarbons, are prepared by contacting the monomer or monomers to be polymerized simultaneously or sequentially with an anionic polymerization initiator such as group IA metals, their allyls, amides, silanolates, naphtalides, biphenyls or anthracenyl derivatives. It is preferred to use an organo alkali metal (such as lithium or sodium or potassium) compound in a suitable solvent at a temperature within the range from about xe2x88x92150xc2x0 C. to about 150xc2x0 C., preferably at a temperature within the range from about xe2x88x9270xc2x0 C. to about 100xc2x0 C. Particularly effective anionic polymerization initiators are organo lithium compounds having the general formula:
RLin
wherein R is an aliphatic, cycloaliphatic, aromatic or alkyl-substituted aromatic hydrocarbon radical having from 1 to about 20 carbon atoms and n is an integer of 1 to 4. The organolithium initiators are preferred for polymerization at higher temperatures because of their increased stability at elevated temperatures.
Polyester polymers would be made by anionic polymerization of a cyclic ester such as a lactone. Caprolactone is frequently used. Polyether polymers would be made by anionic polymerization of a cyclic ether such as an epoxide. Ethylene oxide is frequently used. Polycarbonate polymers would be made by anionic polymerization of a cyclic carbonate. The cyclic carbonate of 1,3-propanediol may be used.
Functionalized polydiene polymers, especially terminally functionalized polybutadiene and polyisoprene polymers, optionally as copolymers, either random or block, with styrene, and their hydrogenated analogs are preferred for use herein. Especially preferred are polybutadiene diols. Such polymers are made as generally described above. One process for making these polymers is described in U.S. Pat. No. 5,393,843 which is herein incorporated by reference.
Using a polydiene diol as an example, butadiene is anionically polymerized using a difunctional lithium initiator such as the sec-butyllithium adduct of diisopropenylbenzene as an example. The living chain ends are then capped with a capping agent such as described in U.S. Pat. Nos. 4,417,029, 4,518,753, and 4,753,991, which are herein incorporated by reference. There are many multilithium initiators that can be used herein. The di- s-butyllithium adduct of m-diisopropenylbenzene is preferred because of the relatively low cost of the reagents involved and the relative ease of preparation. Diphenylethylene, styrene, butadiene, and isoprene will also work well to form dilithium (or disodium) initiators by the reaction: 
Still another compound which will form a diinitiator with an organo alkali metal such as lithium and will work herein is the adduct derived from the reaction of 1,3-bis (1-phenylethenyl)benzene (DDPE) with two equivalents of a lithium alkyl: 
Related adducts which are also known to give effective dilithium initiators are derived from the 1,4-isomer of DDPE. In a similar way, it is known to make analogs of the DDPE species having alkyl substituents on the aromatic rings to enhance solubility of the lithium adducts. Related families of products which also make good dilithium initiators are derived from bis[4-(1-phenylethenyl)phenyl]ether, 4,4xe2x80x2-bis(1-phenylethenyl)-1,1xe2x80x2-biphenyl, and 2,2xe2x80x2-bis[4-(1-phenylethenyl)phenyl]propane (See L. H. Tung and G. Y. S. Lo, Macromolecules, 1994, 27, 1680-1684 (1994) and U.S. Pat. Nos. 4,172,100, 4,196,154, 4,182,818, and 4,196,153 which are herein incorporated by reference). Suitable lithium alkyls for making these dilithium initiators include the commercially available reagents (i.e., sec-butyl and n-butyl lithium) as well as anionic prepolymers of these reagents, polystyryl lithium, polybutadienyl lithium, polyisopreneyl lithium, and the like.
The polymerization is normally carried out at a temperature of 20 to 80xc2x0 C. in a hydrocarbon solvent. Suitable solvents include straight and branched chain hydrocarbons such as pentane, hexane, octane and the like, as well as alkyl-substituted derivatives thereof; cycloaliphatic hydrocarbons such as cyclopentane, cyclohexane, cycloheptane and the like, as well as alkyl-substituted derivatives thereof; aromatic and alkyl-substituted derivatives thereof; aromatic and alkyl-substituted aromatic hydrocarbons such as benzene, naphthalene, toluene, xylene and the like; hydrogenated aromatic hydrocarbons such as tetralin, decalin and the like; linear and cyclic ethers such as dimethyl ether, methylethyl ether, diethyl ether, tetrahydrofuran and the like. The capping reaction is carried out in the same solution and usually at about the same temperature as the polymerization reaction, as a matter of convenience.
The general class of capping agents useful herein which form strongly associating chain ends and cause gelation are those which form alkali metal-O or alkali metal-N (preferably, LiO and LiN) bonds. Specific capping agents which are highly useful herein include ethylene oxide and substituted ethylene oxide compounds, oxetane and substituted oxetane compounds, aldehydes, ketones, esters, anhydrides, carbon dioxide, sulfur trioxide, aminating agents which form lithium imides, especially imines, and suitable reactive amine compounds like 1,5-diazabicyclohexane as described in U.S. Pat. No. 4,816,520 which is herein incorporated by reference. At least 0.1 mole of capping agent per mole of polymer chain end is necessary to give sufficient functionalization for most applications. It is preferred that from 1 to 10 moles of the capping agent per mole of polymer chain end be used in the capping of the polymer although the upper limit is only a practical one determined by cost benefit.
At this point in the process, the polymer forms a gel. A trialkyl aluminum compound is then added to this gel which then dissipates. The alternative process involves adding the trialkyl aluminum compound to the polymer mixture before the alkali metal reacts with the gel-forming functionality to form a gel. It may be added before, during, or after polymerization before the addition of the capping agent. In these cases, no polymer gel forms. If the trialkyl aluminum is added before or during polymerization, then less than a molar ratio of Al:Li of 1:1 should be added because the polymerization will stop if the ratio reaches 1:1. In yet another alternative, the trialkyl aluminum compound is added at the same time as the capping reagent. It may be premixed with the capping agent or just added to the reactor at the same time as the capping reagent. In this process, no polymer gel forms. Using triethyl aluminum as an example, it is believed that the mechanism of these two processes, adding the trialkyl aluminum reagent either before or after capping, is as follows: 
As described above, gel is avoided or removed by addition of a trialkyl aluminum compound. It is important that the chain end retains activity for nucleophilic substitution reactions after the xe2x80x9catexe2x80x9d complex has formed. Even after the trialkyl aluminum reagent has been added and the xe2x80x9catexe2x80x9d complex has formed, the chain end is still capable of further reaction. The trialkyl aluminum compounds used in the present invention are those wherein the alkyl groups contain from 1 to 10 carbon atoms. Preferred trialkyl aluminum compounds are triethyl aluminum, trimethyl aluminum, tri-n-propylaluminum, tri-n-butylaluminum, triisobutylaluminum, tri-n-hexylaluminum, and trioctyl aluminum because these reagents are readily available in commercial quantities. Triethylaluminum is most preferred as it is least expensive on a molar basis.
The molar ratio of the trialkyl aluminum compound to the polymer chain ends is generally at least 0.1:1, preferably 0.33:1 and most preferably 0.66:1 to 1:1 since this results in a freely flowing solution. If it is less than 0.1:1, then the level of reduction in gel is too low to give an observable reduction in either the shear stress or the viscosity of the solution. If the ratio is more that 1:1, then the cost goes up unnecessarily but the advantages are still achieved. It is advantageous to be able to use less aluminum for cost purposes.
This invention is also applicable in situations wherein an existing polymer is to be functionalized or wherein it is desired to convert the functionality of an already functionalized polymer using one of the gel-forming capping agents described herein.
For example, it is known to functionalize hydrogenated styrene-butadiene-styrene (SBS) block copolymers by first lithiating them by reaction with RLin in the presence of a promoter such as triethylamine or tetramethylethylenediamine (TMEDA) as described in U.S. Pat. Nos. 4,868,243 and 4,868,245 which are herein incorporated by reference. A number of reactive Li+ sites are formed in the styrene blocks. If these are reacted, for example, with CO2, strongly associated gel forms. It may be broken by addition of trialylaluminum to the gel or prevented by such addition prior to addition of the CO2 as described above.
Also, an existing polyol such as polybutadiene diol, for example, can be reacted in a hydrocarbon solution such as cyclohexane with KH to form the potassium alkoxide. Potassium alkoxides are known to rapidly polymerize ethylene oxide which would afford a route to a block copolymer having polyethylene oxide end blocks and a polybutadiene center block. At the start of such a synthesis, upon reaction of the polyol with the KH, a gel will form. Trialkylaluminum will break up the gel or prevent its formation as described above.